Preheat oven to 450°F. Wash and cut potatoes in half or quarters, depending on size. Put oil on a 15-1/2-by-10-1/2-by-2-inch baking pan. Heat 3 minutes on the lowest oven rack. Add potatoes and bake for 10 minutes. Carefully turn with a spatula and sprinkle with rosemary. Bake 10 minutes more or until potatoes are tender. Serve immediately.